In the transmission of fluids through pipelines, processing facilities, and the like, it is necessary to control valves within the system accurately and quickly. For example, in petroleum processing plants or chemical manufacturing facilities integrated control schemes often require various valves throughout the system to be opened and closed in sequence, and to various degrees, depending upon a given set of operating parameters. Those operating parameters may include a desired flow rate, a timed release of fluid, the particular fluid being processed, etc.
As such control systems become increasingly automated, it is imperative that the control valves be positioned accurately and responsively. Any lag time between receipt of a position command signal, and the actual positioning of the valve to the desired location detrimentally affects the process in terms of productivity and profitability.
As a result, systems have been developed for responsively positioning a control valve. For example, in one of my previous inventions, disclosed in U.S. Pat. No. 4,509,403, assigned to the present assignee, a positioner system is provided which employs a single feedback loop for dynamically controlling the position of a valve. In addition, U.S. Pat. No. 5,884,894 discloses a positioning system employing first and second control loops for controlling the position of a spool valve. However, spool valves typically exhibit high leakage, and if designed to exhibit less leakage, result in a relatively large "dead zone" in the operating range of the valve. This detrimentally affects responsiveness of the valve. Moreover, such spool valves need to be manufactured to close tolerances, increasing the cost of the valve.